Best way to “preserve” Wii U eshop downloads?

[Eidolon]

With the impending store closures on both the WiiU and 3DS, i've been planning on getting SD Cards for both, and downloading what I can of interest before the good stuff's gone. I've not had a chance yet, with eveything else getting in the way.

From what I gather, inserting an SD Card in the WiiU's front slot is useless. You have to put one in the back with a USB adaptor, at which point the WiiU treats it as a HDD.

Regarding backing stuff up on both systems - Can I copy everythign to my computer, and then just transfer it back onto the relevant SD cards when I want to use it (as there won't be enough room on the cards to keep everything), or is there more flaffing about needed? Thanks.